if a slot is available, don't launch more (#701)
if a slot is available, don't launch more (#701)

since we were sending a signal before checking if a slot was available, even
in the case of serial calls locally I was seeing 2 containers launch. if we
only send a signal after first checking if a slot is available, this goes
away. 1 usec should not be too offensive of an additional wait, all things
considered here.
fnproject/fn,fnproject/fn,fnproject/fn